taskstimetable plugin: autorefresh when shown times changes

this was extracted from general piwik issue list https://github.com/piwik/piwik/issues/4927

small but nice and useful feature:
autorefresh the timetable overview site from task-timetable plugin everytime the cron is triggered and the shown date+time for next run becomes new (the shown data is wrong)
So this site shows always the right times and dates even if it is opened and none refreshed for a long time e.g. during problem research

*mattab commented on 1 Apr
We could add an automatic refresh every 1 minute or so, which is an easy fix!
hpvd

*hpvd commented on 6 Apr
maybe refreshing it every minute is only the half way.
refreshing it always when things have changed would be really better/more helpful during problem research..

Inizia individuando la pagina di panoramica del plugin task-timetable e il percorso di aggiornamento attivato da cron. Conferma come vengono ottenute la data e l’ora visualizzate della prossima esecuzione, quindi definisci il completamento come l’aggiornamento della panoramica quando questi valori cambiano; nell’issue non viene indicato alcun file di implementazione né alcun test.